This invention generally relates to fluorescent lamps and, more particularly, to an improved fluorescent lamp of predetermined chromaticity which has combined high efficacy and good color rendition for illuminated objects and which is protected from color shifts.
U.S. Pat. No. 3,858,082, dated Dec. 3, 1974 to Thornton discloses various three-component phosphor blends which can be used in fluorescent lamps in order to provide both good color rendition of illuminated objects and a high light output. On embodiment of a phosphor blend which is disclosed in this patent utilizes apatite-structured strontium chlorophosphate activated by divalent europium as a blue-emitting phosphor component, manganese-activated zinc silicate phosphor as a green-emitting phosphor component and yttrium oxide activated by trivalent europium as a red-orange emitting phosphor component. The relative proportions of these components can be varied to provide the lamp with a predetermined correlated color temperature which can vary over a wide range, and lamp correlated color temperatures which vary from 3000.degree. K. to 6500.degree. K. are readily obtainable. The overall performance of such lamps is excellent, but on occasion the green-emitting phosphor component displays a relatively rapid depreciation of light output, particularly in the vicinity of the electrodes, which causes a color shift to occur. Although this normally does not affect the overall performance of the lamps, it is considered objectionable from an aesthetic standpoint.
U.S. Pat. No. 3,937,998, dated Feb. 10, 1976 to Verstegen et al. discloses a three-component phosphor blend for use in fluorescent lamps, in order to provide both good color rendition of illuminated objects and a high light output. The components comprising this phosphor blend are very expensive and to decrease the amount of these expensive components needed, there is disclosed in U.S. Pat. No. 4,088,923, dated May 9, 1978 to Manders an underlayer of relatively inexpensive halophosphate phosphor of the same emission color, so that the expensive phosphor blend components are "worked harder", and less material is required. A two layer phosphor coating is also disclosed in U.S. Pat. No. 3,602,758 dated Aug. 31, 1971 to Thornton et al.
In copending Application Ser. No. 58,574, now U.S. Pat. No. 4,263,530, filed July 17, 1979 by J. Van Broekhoven and R. Corth, and owned by the present assignee, is disclosed a fluorescent lamp which incorporates a phosphor blend of warm-white color which comprises a mixture of apatite-structured calcium fluorophosphate activated by antimony and manganese and yttrium oxide activated by trivalent europium. The resulting lamp has both improved output and color rendering index as well as improved color preference index as compared to a standard fluorescent lamp which incorporates a halophosphate phosphor of the same color temperature.
The so-called halophosphate phosphors have been the standard phosphor used in fluorescent lamps for many years and the basic patent describing these phosphors in U.S. Pat. No. 2,488,733, dated Nov. 22, 1949, to McKeag et al. Such phosphors are normally fabricated as apatite-structured calcium fluorochlorophosphate, activated by antimony or antimony plus manganese, preferably with a small addition of cadmium.
The internationally accepted procedure for standardizing and measuring the color rendering properties of light sources is set forth in the publication of The International Commission on Illumination, identified as Publication CIE No. 13(E-1.3.2) 1965. More recently, a color-preference index has been proposed for rating the performance of light sources in accordance with what the normal observer considers to be the preferred coloration for familiar objects. This color preference index (CPI) is summarized in the Journal of the Illuminating Engineering Society, pages 48-52 (October 1974) article entitled "A Validation of the Color-Preference Index" by W. A. Thornton.